The present invention relates to polymer dispersions in polyester polyols, a process for producing them and their use for the production of polyurethanes, in particular microcellular polyurethanes.
Dispersions of solid, high-molecular polymers in polyols (i.e. polymer polyols) are often used for the production of flexible polyurethane foams. The advantage of this, for example, is that the open-cell nature of these foams is increased and the mechanical properties are improved as a result of the increased hardness. In particular, tear strength, tensile stress and compression set are improved. This allows reduced density to be set while maintaining the properties which are otherwise only achievable at higher density. A significant material saving, and thus cost reduction, can thereby be achieved.
Dispersions of polymers in polyols are known in literature, which discloses, in addition to the dispersions obtainable by converting olefin group-containing monomers into polyols, also other types of dispersions such as, e.g., those that are produced from diamines and polyisocyanates. Equally it is clear, that the polyols used are mostly polyether polyols having molar masses of 1,000 to 10,000 g/mol, and less frequently polyester polyols. One reason for this may be the comparatively high viscosity of the polyester polyols themselves and in particular of dispersions based on polyester polyols, in comparison with corresponding systems based on polyether polyols. Nevertheless there is industrial interest in the dispersions based on polyester polyols, in particular because polyurethane systems produced from them have better mechanical properties in many respects than the corresponding polyether-based polyurethanes.
For aqueous systems used in thermosetting stoving enamels, DE-OS 44 27 227 discloses polyester polyols dispersed in water, which are filled with polymers of olefinic monomers, as one of the system components.
If styrene is used in such systems as a vinylic monomer, otherwise analogous dispersions are less stable as a result of its lower reactivity in comparison with acrylonitrile and a chain transfer speed that is lower on several molecule species. As a result of this, the use of styrene as a radically polymerisable vinylic monomer for the production of dispersions based on polyester polyol, requires the incorporation of grafting sites in or at the end of the polyester molecules. This is particularly true if only styrene is used as a vinylic monomer. Such grafting sites must guarantee the chain transfer of the radically growing polymer molecules while forming covalent bonds and, if possible, maintaining the growing radical chain.
Some examples of such modifications are described in EP-A 250 351. Thus, e.g. the incorporation of maleic acid anhydride into the polyester polyol chain can fulfill this function. EP-A 0 250 351 also discloses a process in which at least one ethylenically unsaturated monomer is polymerised in a polyester polyol with a molar mass of 1,000 to 5,000 g/mol. The polyester polyol thus contains, in addition to the conventional structural elements, polycarboxylic acid and polyalcohol, and also olefinic components, specifically the structural element maleic acid anhydride.
The disadvantage of incorporating such unsaturated polycarboxylic acids or anhydrides, which reduce the free movement of the segments of the polyester chain, is however the associated increase in the viscosity of the polyester polyols or polyester polyol mixtures used. Similarly, the increased concentration of polar ester carbonyl functions resulting from the incorporation of maleic acid into the polyester chain has a viscosity-increasing effect. This increased viscosity further reduces the usability of the already per se higher-viscosity polyester polyols.
In addition to these disadvantages, in industrial practice, the polyester polyols modified with unsaturated structural elements have proved in numerous cases to produce coarse-particle dispersions, which mostly contain particles that are visible to the naked eye and are often difficult to filter.
Therefore, an object of the present invention was to provide an improved process for the production of polymer polyols based on polyester polyols. It was discovered that olefinically unsaturated structural elements can be omitted as a component of the polyester polyol, and the resultant polymer polyols nevertheless have a small-particle size and are stable, if modified polyester polyols containing sulfhydryl groups are used as a stabilizer. Thus, the present invention provides polymer dispersions which contain at least one polyester polyol having one or more sulfur atoms.